(1) Field of the Invention
The present invention relates to an industrial polyester filamentary yarn and a tire cord formed from this filamentary yarn. More particularly, it relates to a polyester filamentary yarn, a tire cord having improved fatigue resistance due to increased thermal dimensional stability and strength, as well as a process for production of this yarn.
(2) Description of the Prior Art
In general, nylon, rayon, polyester etc. fibers are typical examples of the fibers which have been used as reinforcements in rubber tires.
Nylon tire cord has higher strength and toughness than the other materials due to the inherent properties of nylon fiber and has been generally used in bias tires for trucks, buses etc. Rayon tire cord provides a low degree of shrinkage and has good thermal and dimensional stability due to the inherent properties of rayon fiber and has been generally used in high speed radial travelling tires for passenger cars.
Unfortunately, nylon tire cord has poor dimensional stability due to its low modulus characteristics and high shrinkage and further exhibits flat spots due to its low glass transition temperature. Rayon tire cord also has low modulus characteristics and exhibits a sharp decrease in strength after the fibers have been formed into a tire cord.
In view of these defects found in both nylon and rayon tire cords, polyester tire cord has been widely used.
Prior art polyester fibers that have been used in tires have benzene rings in their molecular structure, and a rigid molecular chain. Accordingly, tire cord formed from polyester yarn has a good elastic modulus, good fatigue resistance, provides few flat spots, excellent creep resistance and excellent endurance. For these reasons, polyester tire cord has been widely used in radial tires for passenger cars.
However, in spite of the above described merits, conventional polyester tire cords do have a problem: they undergo substantial variation in their properties with temperature due, it is thought, to hysteresis effect. In particular, conventional industrial high strength polyester fibers generally exhibit substantial shrinkage when heated.
Also, when industrial polyester fibers have been incorporated into a rubber matrix of a tire, as the tires rotate during use the fiber is stretched and relaxed during each tire rotation. Further, the internal tire air pressure stresses the fiber, and tire rotation while axially loaded or stressed causes repeated stress variations, particularly on unsmooth surfaces.
Since more energy is consumed during the stretching of a fiber than is recovered during its relaxation, the difference of energy dissipates as heat. This is termed hysteresis or work loss. Significant temperature increases have been observed in rotating tires during use which are attributable at least in part to this fiber hysteresis effect.
The variation in properties caused by heat generation occurs due to moisture and amines contained in conventional rubber solutions used in rubber treatments for producing tire cord, and the observed variation tends to be increased when the content of carboxyl group is high, leading to a significant lowering of strength and fatigue resistance.
In recent years, as radial tires having high performance have been widely developed and used, the demand for polyester tire cords with superior properties, especially properties superior to those obtained with nylon or rayon tire cord, has been increasing. Therefore, research into development of a polyester tire cord having improved fatigue resistance by minimizing the heat generated due to the hysteresis effect has been undertaken.
Prior art methods for improving fatigue resistance of polyester fibers have focused on a chemical method for increasing stability by reducing the content of carboxyl groups in the polyester and a method wherein highly-oriented undrawn yarn produced using a polyester with a relatively low I.V. (intrinsic viscosity), or produced by employing a high-speed spinning process, is drawn.
References directed towards increased chemical stability are Japanese patent laid-open No. Sho. 54-132696 and 54-132697 which disclose the inhibition of deterioration due to thermal decomposition resulting from heat generation by reducing the carboxyl group content of the polyester. By reducing the content of terminal carboxyl groups via copolymerization with or melt-blending in an aliphatic polyester. The increased mobility of amorphous regions effects a reduction in heat generation leading to a reduction in thermal decomposition which effects improved fatigue resistance. But in this method, high crystalline polyester fiber cannot be obtained and the tenacity and initial elastic modulus of the material is always low. Thus, the shrinkage of the resulting fiber is increased and the product obtained is not a high quality tire cord yarn. Also, reducing the content of terminal group by adding a blocking agent has a disadvantage in that the degree of polymerization is lowered and the cost is increased.
References directed to a method of increasing thermal stability by high speed drawing are U.S. Pat. Nos. 4,101,525 and 4,195,052 which disclose an improvement in fatigue resistance by increasing the mobility of the molecular chains in the amorphous region high-speed spinning. In this process, the fatigue resistance is improved but the molecular chain length is irregular and long, and the relaxed molecular chains coexist so that the loss of tenacity is high. Also, a difference in properties between the inner and outer layer of fiber is effected so that the drawability decreases. The resultant variation in physical properties within these regions of the fiber is severe due to the presence of a defective microstructure.
Prior art processes for producing tire cord from yarn include, for example, Japanese patent laid-open No. Sho 61-12952 which discloses a tire cord having a tenacity of at least 7.0 g/d, an absorption peak temperature in the amorphous region of 148.degree.-154.degree. C., a shrinkage of 3.3-5% by spinning a polyester polymer having an intrinsic viscosity of 1.0, a diethylene glycol content of 1.0mol %, a carboxyl group content of 10 eq/10.sup.6 g at a spinning speed of 2,000-2,500 m/min to obtain undrawn yarn, drawing the undrawn yarn at about 160.degree. C., thermally treating at 210.degree.-240.degree. C. and dipping the obtained yarn in a conventional rubber solution.
In addition, U.S. Pat. Nos. 4,101,525 and 4,195,052 disclose a polyester tire cord produced by a process comprising drawing highly oriented undrawn yarn prepared from a high-speed spinning process to obtain highly oriented drawn yarn, specifically multi-drawn yarn comprising 85 mol % polyethylene terephthalate having a denier per filament of 1 to 20 and a work loss at 150.degree. C. of 0.004-0.02, and dipping the multi-drawn yarn in a rubber solution.
In the above methods, tie molecules, which have an important effect upon dimensional stability (especially shrinkage) are oriented. This leads to residual internal stress and finally causes a lowering of the fatigue resistance of the tire cord. In most of the conventional polyester yarns for tire cord, internal stress produces a temperature rise which induces continuous increase of thermal stress. This finally results in poor tire cord fatigue resistance because after the tire cord conversion process (or dipping process) comprising dipping the cord in a rubber solution and thermally treating, an internal stress of about 0.5 g/d usually remains in the tire cord.
Moreover, yarns which are highly oriented drawn yarns before undergoing the tire cord conversion process have a definite two-phase structure with both crystalline and amorphous regions. When it is dipped in a rubber solution and thermally treated, deterioration of the crystalline regions occurs and leads to a lowering of strength.
In addition, Japanese patent laid-open No. Sho. 61-146876 discloses a process for producing a polyester tire cord by spinning yarn with a small mass flow rate per spinneret capillary to attain a relatively high Spin-Draw Ratio, thereby obtaining a highly oriented undrawn yarn at a relatively low spinning rate, producing a high strength yarn followed by dipping it in a rubber solution and thermally treating it at a temperature higher than 220.degree. C. This process has a disadvantage in that the beneficial properties of the polyester yarn are lost in the twisting and dipping process due to deterioration of the crystalline portions of the yarn by heat, and the final dipped cord has rather poor properties.
Japanese Patent laid-open No. Sho. 54-77794 discloses a process which comprises treating polyester drawn yarn with an epoxy resin compound prior to dipping in a rubber solution but this process did not solve the above-described problems.
The present invention has been developed to solve the above described problems of the prior art. According to the present invention, the two problems of i) lowering of fatigue resistance due to residual internal stress by high-speed spinning and ii) lowering of strength due to deterioration of the crystalline portions on dipping in a rubber solution can be solved based upon the points described below.
Polyester yarn having a high crystallinity undergoes a high degree of thermal hysteresis and, accordingly, has a high thermal stress. Thus, it tends to undergo a lowering of strength, elastic modulus or strength conversion efficiency due to formation of folded crystals and in particular, from unconstrained molecular chains in the amorphous regions present during recrystallization which subsequent heat treatments, such as dipping process etc., cause. In addition, though a high crystalline polyester yarn itself may exhibit good thermal stability, dimensional stability and fatigue resistance, the yarn has a definite two phase microstructure which may effect a rapid growth of crystal size or long period growth upon subsequent heat treatment so that the fatigue resistance initially exhibited by the yarn itself can not be obtained after it is twisted and subsequently heat-treated.
Conventional tire cords which have been used as reinforcers in tire production exhibit a shrinkage of at least 10% when subjected to a high temperature. Moreover, when they have been incorporated into the rubber matrix of a tire, the repeated fatigue movements such as stretching, compression and flexing lower the inherent properties of the fiber such as strength, elastic modulus and toughness. Furthermore, the poor fatigue resistance results in bad tire uniformity.
The present inventors have directed their research towards improving the prior art methods of producing a polyester yarn for tire cord which has excellent overall physical properties like strength, and, at the same time, which has a high strength conversion efficiency and excellent dimensional stability leading to excellent fatigue resistance when used even after having been subjected to a cord conversion process and then incorporated into a rubber matrix. As a result of this research, the present invention has been achieved. Thus, whereas most of the prior art methods comprise producing a polyester yarn having a stable two-phase structure of crystalline and amorphous portions or regions and then simply dipping it into a rubber solution to obtain a final tire cord, the present invention comprises producing a polyester yarn having a three-phase structure of crystalline, amorphous and mesomorphous portions and thereafter subjecting it to recrystallization during a dipping process to obtain a tire cord having a stable two-phase structure.
As the mesomorphous portions present in the yarn are crystallized while being subjected to heat in a dipping process, crystals with a 10% smaller size than the crystals obtained in prior art methods are produced and the present yarn in cord form provides dimensional stability by developing a network structure with uniformly formed crystalline and amorphous portions, and has a high elastic modulus due to the minimization of the formation of folded crystals during recrystallization thereby increasing the content of the strained tie molecular chains which interlink crystals.
Moreover, the present inventors have discovered particular spinning and drawing processes which achieve the above characteristics. Consequently, the process conditions necessary to produce an excellent polyester filamentary yarn have been designed. In more detail, a undrawn yarn is produced which has highly oriented molecular chains in amorphous state such that crystalline diffraction by X-ray is not observed, thereafter the undrawn yarn is drawn at a low draw ratio and a low temperature (below the crystallization temperature) so as to minimize the strain of molecular chains in amorphous regions induced by drawing, and then subjected to thermal treatment and relaxing at a low temperature so that no further crystallization proceeds. The filamentary yarn is then dipped into a rubber solution and thermally treated at certain temperature and tension conditions enabling recrystallization to occur, thereby obtaining a final polyester tire cord.